This invention relates to improvements in a thermoplastic resin composition and a resinous material including the thermoplastic resin composition in order to obtain both a low frictional resistance and a high wear resistance for a sliding member such as a shoe of a chain guide or a chain tensioner in a chain system of an internal combustion engine.
In recent years, application of thermoplastic resin has advanced to sliding members such as bearings, rollers and gears from the viewpoints of lowering noise generated with sliding movement, weight-lightening of the sliding members and providing a non-lubrication for a sliding section. Additionally, environmental conditions (such as bearing pressure and temperature) under which the sliding members formed of resinous materials are used have become further severe owing to the fact that the sliding members or parts have been required, for example, to become further small-sized. Particularly, the resinous material of gears, pulleys, shoes of chain guides used for an automotive internal combustion engine are required to have a heat resistance at temperatures of 140° C. or higher and a high oil resistance. Accordingly, polyamide resin is selected as the resinous material because it is excellent in heat resistance, oil resistance, mechanical strength and wear resistance.
However, polyamide resin has such a problem as to be high in frictional resistance during sliding movement though it is excellent in various material characteristics as discussed above. In this regard, it is known as a measure to lower the frictional resistance of polyamide resin, to add a solid lubricant such as fluororesin (for example, polytetrafluoroethylene) or molybdenum disulfide into polyamide, as disclosed in Japanese Patent Provisional Publication No. 2002-53761. However, with this measure, in case that the surface roughness of an opposite member (in sliding contact with the sliding member) is larger or that a sliding speed of the sliding member is higher, the solid lubricant removes from polyamide resin as a matrix thereby forming a removed section which serves as a starting point of occurrence of pitting wear, or a section in which the solid lubricant low in wear resistance exists partially wears. As a result, the wear of the whole sliding member formed of a thermoplastic resin composition is unavoidably promoted.
In order to suppress the wear of the sliding member formed of the thermoplastic resin composition, it is general to add filler such as glass fiber into the composition. However, it may occur according to sliding conditions that the filler fiber removed from the composition is supplied to the sliding surface so as to serve as an abrasive, thereby promoting the wear of the sliding member.
Furthermore, a method for reforming a fluororesin has been proposed in Japanese Patent Provisional Publication No. 2000-186162, in which crosslinking is made in the fluororesin by irradiating an ionizing radiation onto the fluororesin upon heating the fluororesin at a certain temperature in a low oxygen concentration atmosphere. However, this method is characterized by continuously carrying out the irradiation of the ionizing radiation in place of a conventional manner of intermittently carrying out the irradiation. Additionally, the fluororesin to be treated is sheet-shaped in this method. Therefore, this Publication merely discloses a production method of a reformed fluororesin, and therefore the technique disclosed in this Publication cannot lead to improvements for providing both high wear resistance and low frictional resistance to a sliding member formed of a thermoplastic resin composition.